The heavy-elements heritage of the falling sky
Alejandra Recio-Blanco, Emma Fernández Alvar, Patrick de Laverny, Teresa Antoja, Amina Helmi, Aurélien Crida
AAstronomy & Astrophysics manuscript no. RecioBlanco c (cid:13)
ESO 2020July 17, 2020 L etter to the E ditor The heavy-elements heritage of the falling sky
Alejandra Recio-Blanco , Emma Fernández-Alvar , Patrick de Laverny , Teresa Antoja ,Amina Helmi , and Aurélien Crida Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Francee-mail: [email protected] Institut de Ciències del Cosmos, Universitat de Barcelona (IEEC-UB), Barcelona, Spain Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The NetherlandsReceived ...; accepted ...
ABSTRACT
Context.
A fundamental element of galaxy formation is the accretion of mass through mergers of satellites or gas. Recent dynamicalanalysis based on Gaia data have revealed major accretion events in Milky Way’s history. Nevertheless, our understanding of theprimordial Galaxy is hindered because the bona fide identification of the most metal-poor and correspondently oldest accreted starsremains challenging.
Aims.
Galactic Archaeology needs a new accretion diagnostic to understand primordial stellar populations. Contrary to α -elements,neutron-capture elements present unexplained large abundance spreads for low metallicity stars, that could result from a mixture offormation sites. Methods.
We have analysed the abundances of yttrium, europium, magnesium and iron in Milky Way satellite galaxies, field halo starsand globular clusters. The chemical information has been complemented with orbital parameters based on Gaia data. In particular, theorbit’s average inclination has been considered.
Results.
The [Y / Eu] abundance behaviour with respect to the [Mg / Fe] turnovers for satellite galaxies of di ff erent masses reveals thathigher luminosity systems, for which the [Mg / Fe] abundance declines at higher metallicities, present enhanced [Y / Eu] abundances,particularly in the [Fe / H] regime between -2.25 dex and -1.25 dex. In addition, the analysis has uncovered a chemo-dynamicalcorrelation for both globular clusters and field stars of the Galactic halo, accounting for about half of the [Y / Eu] abundance spread.In particular, [Y / Eu] under-abundances typical of protracted chemical evolutions, are preferentially observed in polar-like orbits,pointing to a possible anisotropy in the accretion processes.
Conclusions.
Our results strongly suggest that the observed [Y / Eu] abundance spread in the Milky Way halo could result from amixture of systems with di ff erent masses. They also highlight that both nature and nurture are relevant to the Milky Way’s formation,since its primordial epochs, opening new pathways for chemical diagnostics of our Galaxy building up. Key words.
Galaxy: abundances – Galaxy: halo – Galaxy: formation – Galaxy: stellar content
1. Introduction
The most primitive Galactic stars are essential to understand theMilky Way formation. Nevertheless, the robust identification ofaccreted objects is particularly challenging for stars with primor-dial abundances having at most 30 times less metals than theSun ([Fe / H] (cid:46) -1.5). Kinematical or dynamical indications of ac-cretion are insu ffi cient to reveal ancient mergers (Jean-Baptisteet al. 2017). They need to be complemented by chemical diag-nostics (Freeman & Bland-Hawthorn 2002), as the chemical evo-lution of a system strongly depends on its mass. Compared to themassive Milky Way, satellite galaxies generally present signs ofprotracted evolutions, being more metal deficient and showinga variety of chemical patterns that we should retrieve in the ac-creted populations, now mixed with in situ formed stars.The most commonly used chemical diagnostic of accretionis the α -elements (O, Mg, Si, S, Ca, Ti) ratio with respect toiron ([ α / Fe]). Initially enhanced, the [ α / Fe] abundance starts tostrongly decline with metallicity after the supernovae Ia explo-sion rate reaches a maximum (Matteucci & Greggio 1986). Thisproduces a knee in the [ α / Fe] vs. [Fe / H] trend whose locationprovides constraints on the system total mass: the less massivethe system, the more metal-poor is the [ α / Fe] turnover. Unfor- tunately, this accretion diagnostic is not discriminating enoughfor stars in the Galactic halo, with metallicities lower than the[ α / Fe] turnover of most satellite galaxies. As a consequence,metal-poor field stars kinematically proposed to be members ofancient accreted satellites, like Gaia-Enceladus / Sausage (Helmiet al. 2018; Belokurov et al. 2018), have similar [ α / Fe] abun-dances as non-members for [Fe / H] (cid:46) -1.5 dex. They only appearas a separate sequence at higher metallicity (Helmi et al. 2018),hampering also the detection of low mass mergers. Similarly,the population of clusters in the Galactic halo is mostly homo-geneous in their [ α / Fe] abundances (Recio-Blanco 2018).Galactic Archaeology thus needs a new accretion diagnos-tic to understand the primordial stellar populations and, in thiswork, we have used neutron-capture elements to identify it.Contrary to α -elements, neutron-capture elements present unex-plained large abundance spreads for low metallicity stars, thatcould result from a mixture of formation sites. In particular, wehave considered the logarithm of the ratio of a star’s yttriumabundance with respect to its europium one, [Y / Eu]. Approxi-mately 75% of the solar Yttrium was produced (Prantzos et al.2018) by low and intermediate mass asymptotic giant branch(AGB) stars, through slow neutron captures (relatively to the β -decay rates of unstable nuclei). In addition, first peak s-elements Article number, page 1 of 6 a r X i v : . [ a s t r o - ph . GA ] J u l & A proofs: manuscript no. RecioBlanco like Y have a larger contribution from low mass stars than secondpeak elements like Ba. On the other hand, 94% of europium isproduced by massive stars through rapid neutron captures (Bis-terzo et al. 2014). Proposed Eu production sites are neutron starmergers (Rosswog et al. 1999), high energy winds accompany-ing core collapse supernovae explosions (Woosley et al. 1994) ormagneto-hydrodynamical explosions of fast rotating stars (Win-teler et al. 2012). As a consequence, the [Y / Eu] abundance ratiocharacterizes the relative contribution of low-intermediate massstars with respect to high mass stars, being therefore a good in-dicator of the chemical evolution e ffi ciency.
2. Chemical abundances and orbital parameterestimations
The present study relies on several samples of objects: globularclusters and field stars, both from the Milky Way and its satel-lites. We have made use of abundances of europium, yttrium and[Mg / Fe], collected from di ff erent literature works (c.f. Table 1and further details in the Appendix). Concerning the [Y / Eu] un-certainty estimates, we have examined the abundance dispersionof the objects analysed by more than one study, including thedwarf stars database. The mean dispersion in the [Y / Eu] ratiois 0.07 dex, indicating a reasonable agreement between di ff er-ent literature sources. To adopt a conservative value, we havemultiplied that dispersion by 2, adopting a typical error-bar of0.15 dex.The chemical analysis of Milky Way objects has been com-plemented with orbital parameters based on Gaia data (Gaia Col-laboration et al. 2018a). For globular clusters, the orbital parame-ters are taken from Model-2 in Gaia Collaboration et al. (2018b).They have been computed as the average values over 10 Gyrof integration. To this purpose, we used the median values ob-tained from 1000 orbits for each cluster obtained through MonteCarlo realizations of the initial conditions, considering the ob-servational measurements and their errors. In particular, the or-bit’s average inclination has been computed as arccos(Lz / Ltot).In our convention, the orbital inclination is defined from theGalactic plane and comprised between 0 ◦ and 180 ◦ , with pro-grade orbits below 90 ◦ . Error bars in the orbital parameters asso-ciated to model assumptions, have been estimated by comparingthe results obtained with di ff erent Galactic potentials (definedas Model-1, -2, -3 in Gaia Collaboration et al. 2018b). In par-ticular, the dispersion in the orbital inclination (estimated as thethird quantile value of the di ff erences distribution between twomodels) is 6 degrees. In addition to this main dataset of clusterorbits, we have completed the sample with six additional objectsfrom Vasiliev (2019).For our field stars samples, we have derived the orbital pa-rameters using the python package galpy (Bovy 2015). We as-sume the MWPotential14 Milky Way mass model included inthis package. We derived the action parameters through theaction-angle isochrone approximation (Bovy 2014). As input pa-rameters we have used the radial velocities gathered in Simbad,the Gaia DR2 proper motions and the distances from Bailer-Jones et al. (2018). In addition, we have checked the e ff ect ofusing two di ff erent methodologies of the dynamical parametersfor clusters and field stars. To this purpose, we have re-computedthe clusters orbital inclinations using the field stars methodologycalculated the di ff erences with respect to the Model-2 orbitalresults from Gaia Collaboration et al. 2018. The median abso-lute deviation of the orbital inclination di ff erences is 2.5 degrees,confirming the consistency of the two approaches. Finally, we have assessed the impact of the detected Gaiakinematic biases (Schönrich et al. 2019) in our data. For the fieldstar samples, only 18 targets had a few parameters outside theSchoenrich et al. quality cuts and were excluded from the anal-ysis. Regarding the globular cluster data, the Gaia Collaborationet al. (2018b) database is within the quality cuts, and the Vasilievet al. compilation uses literature distances and line-of-sight ve-locities not concerned by the Gaia parallax bias.
3. Chemo-dynamical correlations and abundancespread in the Halo − . − . − . − . − . − . [Fe/H] (dex) − . . . . [ M g/ F e ] ( d e x ) − . − . − . − . − . − . [ Y / E u ] Fig. 1.
Mg abundance with respect to iron as a function of [Fe / H] forstars belonging to low mass satellites (Ursa Minor, Draco and Carina;square symbols) and to higher mass satellites (Fornax, Sculptor and LeoI; circles). Points are colour coded by the stars [Y / Eu] content.
First of all, we have analysed the [Y / Eu] behaviour with re-spect to the [Mg / Fe] turnovers for satellite galaxies of di ff erentmasses. Figure 1 shows the Mg abundance (an α -element) withrespect to iron as a function of [Fe / H] for stars belonging to lowmass satellites (Ursa Minor, Draco and Carina; square symbols)and to higher mass satellites (Fornax, Sculptor and Leo I; cir-cles). The points are colour coded by the stars [Y / Eu] content.It can be observed that higher luminosity systems, for which the[Mg / Fe] abundance declines at higher metallicities, present en-hanced [Y / Eu] abundances, particularly in the [Fe / H] regime be-tween -2.25 dex and -1.25 dex (see the Appendix for a separate[Y / Fe] and [Eu / Fe] analysis).Following the previous result, the observed [Y / Eu] abun-dance spread in our Milky Way could result from a mixture ofsystems with di ff erent masses. If this is the case, the [Y / Eu] in-dicator should be compatible with the commonly used [Mg / Fe]accretion diagnostic, also in our Galaxy. This has already beenobserved in the high metallicity regime (Fishlock et al. 2017),but it is di ffi cult to test in the metal-poor one, where the [ α/ Fe]spread is very low.Fortunately, since the arrival of precise Gaia astrometric data,dynamical information can be used to break down this degen-eracy. Indeed, chemo-dynamical correlations retrieved both inthe [Mg / Fe] and the [Y / Eu] spread could reinforce the [Y / Eu]abundance as a good accretion indicator. To test this possibility,
Article number, page 2 of 6lejandra Recio-Blanco et al.: The heavy-elements heritage of the falling sky − . − . − . − . − . . [ Y / E u ] ( d e x ) a) − . − . − . − . − . − . . [Fe/H] (dex) . . . [ M g/ F e ] ( d e x ) b) Mean orbital inclination . . . . . . N o r m a li z e dnb . o f s t a r s d) . . . . . . . N o r m a li z e dnb . o f s t a r s c) Fig. 2.
Panel a: Yttrium abundance with respect to europium as a function of iron abundance for Milky Way globular clusters (large circles)and field stars (squares for a metal-poor sample (Roederer et al. 2014), diamonds, stars and crosses for an intermediate-metallicity compilation(Fishlock et al. 2017) of high-[Mg / Fe], low-[Mg / Fe] and thick disc stars respectively). The colour code is based on the [Y / Eu] departures fromthe standard value (black line). Panel b: [Mg / Fe] abundance ratio with respect to iron for the previous objects, when available, and a compilationof high transversal velocity stars from APOGEE. The colour code selects groups with di ff erent [Mg / Fe] turnovers (thus parent system masses).Panels c and d: distributions of orbital inclinations for the groups selected with the [Y / Eu] and the [Mg / Fe] criteria, respectively. our Milky Way objects have been classified into three categories,using the [Y / Eu] and the [Mg / Fe] criteria independently (upperand lower panels of Figure 2, respectively): first, objects withdepleted [Y / Eu] values or metal-poor [Mg / Fe] turnovers (redtargets) compatible with low-mass progenitors; second, objectswith intermediate [Y / Eu] abundances or intermediate-metallicity[Mg / Fe] turnovers (green targets) possibly formed in higher-mass systems; and third, targets with enhanced [Y / Eu] valuesor a metal-rich [Mg / Fe] turnover typical of the Milky Way insitu population (blue objects). The [Mg / Fe]-selected samples acthere as control groups testing the [Y / Eu] diagnostic.Panels c and d show the normalized distribution of or-bital inclinations for the three sets of objects, selected eitherwith the [Y / Eu] diagnostic or with the [Mg / Fe] one, respec-tively. Although the two chemical diagnostics target di ff erentobjects (those in common being excluded from panel d his-tograms) and span di ff erent metallicity regimes, the similari-ties between panels c and d distributions are important. Two-sampled Kolmogorov-Smirnov tests between the nine possiblepairs of distributions have been performed to test this similar-ity. The null hypothesis, assuming that the samples come froma population with the same distribution, is rejected for all thepairs except those having the same colour (targeting therefore thesame parent system mass). In particular, depleted [Y / Eu] objectstend to present high orbital inclinations, as targets with a metal-poor [Mg / Fe] turnover. On the contrary, objects with interme-diate [Y / Eu] abundances and intermediate metallicity [Mg / Fe]turnovers display mainly low inclination retrograde orbits. Fi-nally, targets with high [Y / Eu] ratios and metal-rich [Mg / Fe]turnovers show primarily low inclination prograde orbits. As ex-pected, adjacent groups in [Y / Eu] or [Mg / Fe] abundances (red-green and green-blue pairs), partially overlap in their orbital in-clination distributions as a result of abundance uncertainties, but also to the fact that no perfectly separated components seem toexist. In particular, in situ formed objects dynamically heated bypast mergers (e.g. Belokurov et al. 2019; Di Matteo et al. 2019)could also blur the orbital inclination distributions.The above result confirms the coherence of the [Y / Eu]diagnostic with the [Mg / Fe] one, revealing possible chemo-dynamical correlations with two independent chemical indica-tors. To quantify those trends, Figure 3 shows the deviationsin [Y / Eu] and [Mg / Fe] abundances with respect to the average,as a function of orbital inclination. Contrary to the analysis ofFigure 2, no data subsamples are predefined and the consid-ered metallicity regime spans -2.0 ≤ [M / H] ≤ -1.2 dex in bothpanels. The two chemical diagnostics show under-abundancesaround the polar direction (60 ◦ (cid:46) inclination (cid:46) ◦ ) and over-abundances near the plane (prograde objects with inclination (cid:46) ◦ and retrograde objects with inclination (cid:38) ◦ ). The ob-served chemo-dynamical correlations, including both globularclusters and field stars, are more pronounced for the [Y / Eu]abundances than for the [Mg / Fe] ones as expected from theircorresponding abundance spreads in this metallicity regime. Inparticular, the orbital inclination seems to account for about halfof the [Y / Eu] abundance scatter.
4. Conclusions
Although Galactic studies need to be constantly validated in thehuge parameter space of Milky Way populations, the observedchemo-dynamical correlations open new paths of exploration ofour Galaxy formation history. In the light of the previous con-clusions, the heavy elements abundance scatter of the primordialMilky Way possibly results from an amalgam of systems withdi ff erent masses and chemical evolutions. Article number, page 3 of 6 & A proofs: manuscript no. RecioBlanco − . − . . . . . ∆ [ Y / E u ] ( d e x ) a) Orbital inclination (deg) − . − . . . . ∆ [ M g/ F e ] ( d e x ) b) Fig. 3.
Deviations in [Y / Eu] (panel a) and [Mg / Fe] (panel b) abun-dances, with respect to the average, as a function of the orbital incli-nation. No objects in common to the [Y / Eu] and the [Mg / Fe] analysisare included. Average values have been defined by a Theil-Sen linearfit for each abundance trend with metallicity. The [Mg / Fe] analysis isrestricted to -2.0 ≤ [M / H] ≤ -1.2 dex, to reduce the non linear e ff ect ofthe [M / Fe] turnover.
First, objects in polar-like orbits showing underabundancesof [Y / Eu] could result from a composite debris from low massaccretions. Interestingly, polar orbits are also found for more re-cent merger events as the Sagittarius one. This suggests the pos-sible existence of a preferential accretion axis around the po-lar direction, linking the Milky Way to its satellites and deserv-ing further study. In the metal-poor and intermediate metallicityregime, where the [Y / Eu] under-abundances are larger than the[ α / Fe] ones, future large scale heavy-element studies seem cru-cial to distinguish between low-mass accretions and slow rotat-ing debris from more massive mergers.Second, satellite merger debris in retrograde orbits was pre-viously suggested by the analysis of several dynamical overden-sities (e.g. Helmi et al. 2018; Belokurov et al. 2018; Myeonget al. 2019), and attributed to high mass progenitors (Gaia Ence-ladus / Saussage, Sequoia). In our study, the chemical patternsdominating that retrograde regime near the plane are indeed typ-ical of high mass systems, reaching metallicities of -0.5 dex andrelatively high [Y / Eu] abundances. The interplay of this old ret-rograde population with the prograde disc and the slow rotatingaccretion debris is probably an important piece of the Galaxyformation puzzle.Third, a prograde population, showing [Y / Eu] overabun-dances, seems to be present even in the low metallicity regime.It could be the fossil signature of the primitive collapsed Galaxy,probably occupying prograde orbits near the plane, as the moremetal-rich disc. This hypothesis is strengthen by the recent dis-covery of very metal-poor stars with disc like orbits (Sestito et al.2020)In conclusion, both nature and nurture appear to have playeda role to build up the ancient Milky Way, leaving inprints weare starting to decode. Chemical diagnostics, including heavy elements abundances, will certainly be fundamental in the ongoing Gaia revolution.
Acknowledgements.
This work has made use of data from the Eu-ropean Space Agency (ESA) mission Gaia Data Processing andAnalysis Consortium (https: // / gaia), processedby the Gaia Data Processing and Analysis Consortium. (DPAC,https: // / web / gaia / dpac / consortium). Funding for theDPAC has been provided by national institutions, in particular the institutionsparticipating in the Gaia Multilateral Agreement. ARB, PdL and EFA ac-knowledge financial support from the ANR 14-CE33-014-01. TA has receivedfunding from the European Union’s Horizon 2020 research and innovationprogramme under Marie Sklodowska-Curie grant agreement number 745617and also acknowledges funding from the MINECO (Spanish Ministry ofEconomy) through grants ESP2016-80079-C2-1-R (MINECO / FEDER, UE) andESP2014-55996-C2-1-R (MINECO / FEDER, UE). AH acknowledges fundingfrom a Vici grant from the Netherlands Organisation for Scientific Research(NWO). We thank E. Vasiliev for providing his orbital parameters for globularclusters. ARB thanks Vanessa Hill, Sebastian Peirani and Oliver Hahn for usefuldiscussions and Chris Wegg for kindly language corrections.
References
Abolfathi, B., Aguado, D. S., Aguilar, G., et al. 2018, ApJS, 235, 42Allen, C. & Santillan, A. 1991, Rev. Mexicana Astron. Astrofis., 22, 255Bailer-Jones, C. A. L., Rybizki, J., Fouesneau, M., Mantelet, G., & Andrae, R.2018, AJ, 156, 58Belokurov, V., Deason, A. J., Erkal, D., et al. 2019, MNRAS, 488, L47Belokurov, V., Erkal, D., Evans, N. W., Koposov, S. E., & Deason, A. J. 2018,MNRAS, 478, 611Bisterzo, S., Travaglio, C., Gallino, R., Wiescher, M., & Käppeler, F. 2014, ApJ,787, 10Bovy, J. 2014, galpy: Galactic dynamics packageBovy, J. 2015, ApJS, 216, 29Di Matteo, P., Haywood, M., Lehnert, M. D., et al. 2019, A&A, 632, A4Fishlock, C. K., Yong, D., Karakas, A. I., et al. 2017, MNRAS, 466, 4672Freeman, K. & Bland-Hawthorn, J. 2002, ARA&A, 40, 487Gaia Collaboration, Brown, A. G. A., Vallenari, A., et al. 2018a, A&A, 616, A1Gaia Collaboration, Helmi, A., van Leeuwen, F., et al. 2018b, A&A, 616, A12Harris, W. E. 1996, AJ, 112, 1487Helmi, A., Babusiaux, C., Koppelman, H. H., et al. 2018, Nature, 563, 85Irrgang, A., Wilcox, B., Tucker, E., & Schiefelbein, L. 2013, A&A, 549, A137James, G., François, P., Bonifacio, P., et al. 2004, A&A, 414, 1071Jean-Baptiste, I., Di Matteo, P., Haywood, M., et al. 2017, A&A, 604, A106Johnson, C. I., Caldwell, N., Rich, R. M., et al. 2017, ApJ, 842, 24Massari, D., Mucciarelli, A., Dalessandro, E., et al. 2017, MNRAS, 468, 1249Matteucci, F. & Greggio, L. 1986, A&A, 154, 279McWilliam, A., Geisler, D., & Rich, R. M. 1992, PASP, 104, 1193Muñoz, C., Geisler, D., & Villanova, S. 2013, MNRAS, 433, 2006Myeong, G. C., Vasiliev, E., Iorio, G., Evans, N. W., & Belokurov, V. 2019,MNRAS, 488, 1235Nissen, P. E. & Schuster, W. J. 2010, A&A, 511, L10Prantzos, N., Abia, C., Limongi, M., Chie ffi , A., & Cristallo, S. 2018, MNRAS,476, 3432Recio-Blanco, A. 2018, A&A, 620, A194Roederer, I. U., Preston, G. W., Thompson, I. B., et al. 2014, AJ, 147, 136Roederer, I. U. & Sneden, C. 2011, AJ, 142, 22Rosswog, S., Liebendörfer, M., Thielemann, F. K., et al. 1999, A&A, 341, 499Schönrich, R., McMillan, P., & Eyer, L. 2019, MNRAS, 487, 3568Sestito, F., Martin, N. F., Starkenburg, E., et al. 2020, MNRAS, 497, L7Shetrone, M., Venn, K. A., Tolstoy, E., et al. 2003, AJ, 125, 684Suda, T., Katsuta, Y., Yamada, S., et al. 2008, PASJ, 60, 1159Vasiliev, E. 2019, MNRAS, 484, 2832Winteler, C., Käppeli, R., Perego, A., et al. 2012, ApJ, 750, L22Woosley, S. E., Wilson, J. R., Mathews, G. J., Ho ff man, R. D., & Meyer, B. S.1994, ApJ, 433, 229 Article number, page 4 of 6lejandra Recio-Blanco et al.: The heavy-elements heritage of the falling sky
Appendix A: Complementary information onliterature abundances
The adopted references for the abundances of the di ff erent ele-ments and populations analysed in this work are summarized inTable 1.A. The study of globular clusters chemical abundancesis currently confined to heterogeneous compilations from di ff er-ent groups. Nevertheless, despite these words of caution, clustersbenefit today from several decades of e ff orts in chemical abun-dance estimations. The analysed Milky Way field stars abun-dances come from three di ff erent compilations: a photometricselection of metal-poor stars (Roederer et al. 2014), a study ofheavy-element abundances for high- α and low- α stars at inter-mediate metallicity (Fishlock et al. 2017) and a selection of hightransversal velocity stars from the APOGEE survey (Abolfathiet al. 2018). When considering the field star homogeneous abun-dances from Roederer et al. 2014, we only take into account starswith abundances estimated from 3 or more lines in order to selecta high quality sample. We do not consider stars for which onlyupper limits were provided. The APOGEE sample is composedof Gaia DR2 stars with parallax > <
15 mag andVtot >
180 km / s. Our final sample comprises 972 objects withAPOGEE DR14 [Mg / Fe] abundances. In addition, the chemicalabundances of Milky Way satellites have been analyzed using acompilation with metallicities [Fe / H] < -0.5 dex, obtained fromthe SAGA database (Suda et al. 2008). We gather stars with Y,Eu and Mg abundance determinations, excluding those with onlyupper limits, carbon-enriched stars (defined as [C / Fe] < / H] < -1.0 dex) and objects reported as binaries.To better understand the [Y / Eu] behaviour, a separate studyof [Eu / Fe] and [Y / Fe] abundance trends with [Mg / Fe], for MilkyWay satellites of di ff erent luminosities can be performed. Fig-ure A.1 shows the Mg abundance with respect to iron as a func-tion of [Fe / H] for stars belonging to low mass satellites (UrsaMinor, Draco and Carina; square symbols) and to higher masssatellites (Fornax, Sculptor and Leo I; circles). A colour code onthe [Eu / Fe] and [Y / Fe] abundances is used for panels a and b ,respectively. Stars showing high [Mg / Fe] values present lower[Eu / Fe] abundances than those of similar metallicity with lower[Mg / Fe] values. As a consequence, stars with [Eu / Fe] abun-dances lower than about 0.5 dex display low-[Mg / Fe] abun-dances only for metallicities higher than around -1.75 dex, sug-gesting a faster chemical evolution of their parent systems. Con-versely, at a given metallicity, higher [Mg / Fe] stars tend to haveslightly higher [Y / Fe] values than lower [Mg / Fe] stars. This sug-gests that lower mass systems tend to present higher [Eu / Fe] en-richments and lightly lower [Y / Fe] abundances than more mas-sive ones, conducting to higher [Y / Eu] ratios as shown in Fig-ure 1.
Article number, page 5 of 6 & A proofs: manuscript no. RecioBlanco − . − . − . − . − . − . [Fe/H] (dex) − . . . . [ M g/ F e ] ( d e x ) a) . . . . . . . . [ E u / F e ] − . − . − . − . − . − . [Fe/H] (dex) − . . . . [ M g/ F e ] ( d e x ) b) − . − . − . − . . . [ Y / F e ] Fig. A.1.
Mg abundance with respect to iron as a function of [Fe / H] for stars belonging to low mass satellites (Ursa Minor, Draco and Carina;square symbols) and to higher mass satellites (Fornax, Sculptor and Leo I; circles). Points are colour coded by the stars [Eu / Fe] content (panel a)and by their [Y / Fe] abundance (panel b).
Table A.1.
Adopted references for yttrium, europium and magnesium abundances
Population [Y / Fe] & [Eu / Fe] references [Mg / Fe] references